Read more... - Johannes Gutenberg-Universität Mainz

Read more... - Johannes Gutenberg-Universität

Earth and Planetary Science Letters 359–360 (2012) 1–13Contents lists available at SciVerse ScienceDirectEarth and Planetary Science Lettersjournal homepage: www.elsevier.com/locate/epslShear heating induced lithospheric-scale localization:Does it result in subduction?Marcel Thielmann a,n , Boris J.P. Kaus b,ca Geophysical Fluid Dynamics, Institute of Geophysics, ETH Zurich, Sonneggstrasse 5, CH-8092, Zurich, Switzerlandb Institute of Earth Sciences, JohannesGutenberg-UniversitätMainz, Germanyc Department of Earth Sciences, University of Southern California, USAarticle infoArticle history:Received 26 January 2012Received in revised form1 October 2012Accepted 3 October 2012Editor: Y. RicardKeywords:subduction initiationnumerical modelingscalingshear heatingabstractEven though it is a well-established fact that the Earth is currently in a plate-tectonics mode, thequestion on how to ‘‘break’’ lithospheric plates and initiate subduction remains a matter of debate.Here we focus on shear heating as a potential mechanism to cause lithospheric shear localizationand subsequent subduction initiation in oceanic plates. It is shown that shear heating under someconditions (i) facilitates the formation of a lithospheric-scale shear zone and (ii) is capable ofstabilizing a lithospheric-scale shear zone, thus creating the necessary condition for subductioninitiation to occur. Furthermore, we demonstrate that not only the localization process is ofimportance, but also the post-localization stage, where rapidly growing convective instabilitiesmight prevent an incipient subduction zone from developing.We develop scaling laws for both the localization and the post-localization stages. In the case oflithospheric localization, the characteristic length scale equals a quarter of the dominant foldingwavelength in the lithosphere, thus of the order of the competent layer thickness. This shows thatoceanic lithosphere develops an intrinsic perturbation length scale that is largely independent ofsmaller-scale heterogeneities. We compare the subduction initiation potential of wet olivine and dryolivine rheologies and find that, for Earth-like conditions, a dry olivine rheology is more likely toresult in subduction initiation. A large plate age does not always increase the potential for subductioninitiation, as it increases the potential for convective instabilities to occur. Instead, an optimal plateage exists for shear heating induced subduction initiation, which is around 40 Ma for a wet olivinerheology.& 2012 Elsevier B.V. All rights reserved.1. IntroductionThe physical mechanisms that result in subduction initiationof a largely homogeneous lithospheric plate have been a matter ofresearch for the last decades. Generally it is thought that oceaniclithosphere densifies as it cools, therefore becoming increasinglynegatively buoyant as it ages. At some point, this negative buoyancyshould be large enough to prompt the oceanic lithosphere tosink into the mantle, therefore initiating subduction. However, ithas been shown with simple analytical models (assuming either apurely elastic lithosphere with a prescribed fault plane McKenzie,1977, or assuming an additional ductile layer beneath the elasticpart of the lithosphere Mueller and Phillips, 1991) that buoyancyforces are too small to bend the lithosphere and overcome thefrictional stresses in the fault and that therefore external forcesn Corresponding author.E-mail address: marcel.thielmann@erdw.ethz.ch (M. Thielmann).are required to initiate subduction. This argument not only holdsfor intra-oceanic settings, but also for passive margins. Cloetinghet al. (1989), for example, even argue that subduction initiation ismore likely for young lithospheres, as the strength of an agingoceanic lithosphere increases faster than its negative buoyancy.The reluctance of an oceanic lithosphere to enter subduction canalso be observed in numerical models on both lithospheric andglobal scale. To obtain plate-like behavior, global mantle convectionmodels usually employ a yield stress (e.g. Tackley, 2000;van Heck and Tackley, 2008). Yet, numerical models need to useyield stresses of 100–200 MPa, far below experimental values ofaround 1 GPa for the mantle lithosphere (e.g. Kohlstedt et al.,1995). The presence of cratons in numerical models increases theyield stress slightly (Rolf and Tackley, 2011), but the requiredyield stresses still remain significantly lower than laboratoryvalues. In contrast, lithospheric-scale numerical (and even analog)models usually prescribe a weak zone that decouples adjacentplates and thus facilitates subduction initiation (e.g. Hall et al.,2003; Mart et al., 2005; Gerya et al., 2008; Baes and Govers,0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.epsl.2012.10.002

You are Johannes Gutenberg, and your assistant is Peter Schoeffer. You have managed to acquire an investor by the name of Johann Fust. He is not a patient man, yet he has shown quite an interest in your discovery. With the investment given to you, you set up a workshop in Mainz and begin to take on the task of finding a way to print books faster than those that handwrite them. It will depend on your knowledge to determine if your ambitious goal of printing over 180 Bibles in a 5-year period will succeed. Answer 20 trivia questions to see how Johannes Gutenberg will manage. After the initial story, enjoy a crossword and word search puzzle. Thank you for reading a Trivia Gamebook!